Conditions  of  Sensibility  of  Photo -Electric 
Cells  with  Alkali  Metals  and  Hydrogen 


TJ'sriV.QF 


CONDITIONS  OF  SENSIBILITY  OF  PHOTO-ELECTRIC 
CELLS  WITH  ALKALI  METALS  AND  HYDROGEN 


BY 


JACOB  GARRETT  KEMP 

A.  B.  University  of  Illinois,  1906 
A.  M.  University  of  Illinois,  1910 


THESIS 

Submitted  in  Partial  Fullfillment  of  the  Requirements  for  the 

Degree  of 
DOCTOR  OF  PHILOSOPHY 

IN  PHYSICS 

IN 

THE  GRADUATE  SCHOOL 

OP  THE 

UNIVERSITY  OF  ILLINOIS 
1912 


11131 


UNIVERSITY  OF  ILLINOIS 


THE  GRADUATE  SCHOOL 


May  11 ,  19 12 , 


1  HEREBY  RECOMMEND  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

Jacob Garrett  Kemp 

ENTITLED  "J^.PI^^^^J^io 9^  .3..?^^  i.^  i  IjAl.  9^"  ® '^J^^^^ 

ri  th  Alakli  MstaLls  and  Hydrogen 
BE   ACCEPTED   AS   FULFILLING    THIS    PART    OF    THE   REQUIREMENTS  FOR  THE 


DEGREE  OF 


Cog  tor  of  E!!?..i...^.o..?9.P.^.Y.  ^J' 


Head  of  Department 


Final  Examination 


Committee 


on 


UlUC 


1 . 

"Conditions  of  renflihility  of  Photo-El ec trie  Cells  with 
Alkali  "et.ils  and  Hydrogen." 

Introduction. 

Under  certain  con-Utions  all  metals  emit  electricity  v;hen  acted 
upon  by  light  waves.     This  pheno/r.enon  is  called  the  "Photo-Electric 
Effect",   i.e.,   the  Light-Electric  Effect.     The  intensity  of  this  ef- 
fect is  largest  for  those  metals  vhich  are  most  electro-positive; 
i.e.,   those  metals  which  give  off  electrons  m^st  readily.     The  order 
of  photo-electric  strengths  for  the  metals  as  given  by  J.J. Thomson^ 
is  as  follows:-  caesium,   rubidium,  potassium,  potassium-sodium  alloy, 
sodium,  lithium,  magnesium,   thallium  and  zinc.     For  copi^er,  platinum, 
lead,  iron,  cadmium,  carbon  and  mercury  tlie  photo-electric  effect  is 
very  weak.     The  order  of  the  metals  for  this  e^'fect  is  precisely  the 
same  as  that  in  the  "^^olta  series  for  contact-electricity. 

Ey  means  of  I>Jaxwell's  electromagnetic  theory  of  light,  the  elec- 
tric action  of  a  beam  of  light  incident  upon  a  metal  surface  may  be 
partly  explained.     In  a  beam  of  light  plane  polarized  at  right  angles 
to  the  plane  of  incidence,   there  is  an  electric  force  vvith  a  compo- 
nent normal  to  the  reflecting  surface,     ^len  light  is  polarized  in 
the  plane  of  incidence,   the  electric  force  is  parallel  to  the  re- 
fleeting  surface.     Elster  and  Geit^l*^  made  the  very  interesting  dis- 
covery that  v/hen  the  plane  of  polarization  is  at  right  angles  to  the 
plane  of  incidence  the  photo-electric  current  is  a  maxiinum  for  any 
given  ar^gle  of  incidence.     Furthermore,   they  showed  that  when  the 
angle  of  incidence  is  about  60°  the  effect  is  a  maximum;  and,  when 

1 .  "Conduction  of  Electricity  through  Cases",  pac^e  351. 

^,     Weid.Ann.  LII  p.433,1824;  I,T  p.684, 1895;  LXI  p.  445, 1807 . 


Digitized  by  the  Internet  Archive 

in  2013 


http://archive.org/details/conditionsofsensOOkemp 


the  quantity  of  li^ht  ataorbed  is  a  n,aximurri  the  effect  ie  a  maximui:.. 

r.Fohl^,  and  R.Tohl    n  .1  r . Tr ing&hei rr,^,  have  shown  in  two  papers 
that  there  are  two  effects,  a  normal  and  a  selective  effect,  ouper- 
imposed  upuii  each  other  wh^n  X)olari7.ed  light  acts  upon  a  metal.  In 
the  nori.ial  effect  the  current  incr pases  with  the  frequency  of  the  in- 
cident light  and  the  orientation  of  the  plane  of  polarization  is  of 
practically  no  influence  except  in  so  far  as  the  absorption  of  light 
depends  upon  it.     The  selective  e'^fect  Joes  net  appear  by  itself, 
but  is  always  superimposed  upon  the  norrnal  effect.     The  curves  in 
Fig.  1,    below  show  the  two  effects  as  they  are  superirapoKed  upon  each 
oth^r  for  a  particular  angle  of  orientation  of  the  plane  of  polariza- 
tion. 


A  4-00/^ 

FIO.  1. 


Current  is  plotted  as  ordinates  and  wave  lengths  of  incident 
light  as  abscissae.     The  curve  S  shows  the  selective  effect  and  the 
curve  TJ  shows  the  normal  effect. 

The  selective  effect  has  the  following  features:- 
1)  it  is  restricted  to  a  short  interval  of  wave  length. 

the  pho to-^elec trie  current  reaches  a  maximum  for  a  particular 

wave  length  independent  of  the  orientation  of  the  plane  of 

1.  R.Pohl.  "^T-erh-der  Phy.G.  p,350,  19  09. 

2.  K.Pohl  ^  P,Pringsheim,  "^'erh-der  Phys.O.  p.  474,  1911. 


3 

polarization. 

3)   the  Gurr'^nt  la  mainly  determined  by  the  orientation  of  the  plane 
o  f  polarization . 

Thus  it  is  seen  that  the  oriantation  of  the  plane  of  polririza tion 
does  not  chanse  the  position  of  the  curve  IT,  but  it  does  cause  the 
curx-e  R  to  be  shifted  parallel  to  itself  in  the  ^rsrtical  direction. 
The  highest  position  of  the  curve  P!  above  Ihe  horizontal  exists  for 
the  plane  of  polarization  perpendicular  to  the  plane  of  incidence. 
The  curve  IT  remains  in  about  the  same  position  except  for  slight 
variations  due  to  the  change  in  the  amount  of  light  absorbed  for 
different  conditions.     Thus  it  is  seen  that  when  the  plane  of  polar- 
ization is  perpendicular  to  the  plane  of  incidence,   the  photo-elec- 
tric current  is  a  maximum  and  due  mainly  to  the  selective  effect. 
Therefore,   the  photo-electric  current  is  a  maximum  for  any  given 
condition  when  the  electric  component  of  the  incident  beam  of  light 
normal  to  the  metal  surface  is  in  the  region  of  its  maximum  value. 

The  intensity  of  light  is  the  amount  of  energy  passing  through 
unit  area  normal  to  the  direction  of  propagation  in  unit  time;  i.e., 
it  is  proportional  to  the  mean  square  of  the  amplitude,  therefore, 
to  the  mean  square  of  the  electric  component.     Lenard"^  found  that 
the  velocity  of  projection  of  electrons  from  metals  due  to  incident 
ultra-violet  light  is  independent  of  the  intensity.     However,  he 
found  that  the  number  of  electrons  emitted  proportional  to  the  in  - 
tensity  of  the  incident  light  while  the  velocity  of  each  electron 
depends  only  upon  the  nature  of  the  illuminated  s\irface.     In  other 
words,   the  velocity  of  the  emitted  electrons  is  a  function  of  the 
energy  absorbed  or  is  proportional  to  the  square  root  of   the  maximum 
voltage  to  v;hich  the  mietal  rises  when  light  is  incident  upon  it. 

1.  Ann.der  Phys.  "IT.  p.  149,  19  03. 


And  the  nuniber  of  electrons  emitted  per  unit  time,  or  the  current,  is 
proportional  to  the  mean  square  of  the  normal  component  of   the  elec- 
tric force  in  the  incident  light  waves  acting  upon  the  metal  surface. 

Ladenburg^  has  shown  that  the  photo-electric  effect  varies  with 
the  thickness  of  the  metal  layer  until  10""*  cm.  is  reached  when  the 
effect  is  independent  of  the  thickness. 

From  the  abo^^e  value  of  10""*  cm.  J  .J .  Thomson^  has  calculated, 
that  if  a  photo-electric  s+.ream  of  1C~^^  coulorubs  per  second  per 
square  centimeter,  which  is  one  of  more  than  average  intensity,  be 
flowing  from  a  metal  surface  which  is  10""^  cm.   thick,  more  than  300 
years  would  elapse  before  +he  character  of  the  surface  would  be 
changed.     This  calculation  gives  some  idea  of  the  constancy  of  the 
photo-electric  property  of  a  metal  under  the  action  of  light.  But 
there  are  other  changes  which  are  liable  to  take  place,  due  to  the 
influence  of  temperature  changes  and  the  action  of  remnant  gases. 

Elster  and  Geitel^,  working  with  potassium  photo-electric  cells, 
found  the  rate  of  discharge  of  electrons  or  the  current,   from  the 
surface  of  the  metal  to  be   lirectly  proper tional ' to  the  intensity 
of  the  incident  light  for  small  ranges  of  light  intensities. 
Fdchtmeyer'*  found  that  the  current  from  a  sodium  surface  at  zero 
potential  is  strictly  proportional  to  the  light  intensities  between 
C.007  C.f.  and  0.5  O.f.      His  cell  was  made  of  a  glass  tube  with 
sodium  upon  one  electrode  and  platinum  wire  for  the  other  in  a  very 
high  vacuum. 

In  1910,  Elster  and  Geitel^  made  photo-electric  cells  using  the 

1.  Ann.der  ?hy.  XII.  p. 558,  1903. 

2.  Conduction  of  Electricity  through  Gases,  p. 278. 

3.  Ann.der  Fhy,   48,  p. 625,  1893. 

4.  Phys.  Rev.  29,  p. 71,   19  09. 

5.  Phys..Seitschr .     11.  April,  1910. 


  g 

metals  caeoiun),  rulidiuin,  potauoiuin  .'-■'Iju  sodiu/n  (aa  the  cathode)  in 
hydrogen  gtxQ ,     The  electrode  in  contact  with  the  metal,   that  is  the 
cathode,  was  connected  to  thi-  negative  terminal  of  a  battery  of  3C0 
or  400  volts  while  a  resistance  of  30OO  ohrns  arid  a  galvanometer  was 
placed  in  series  with  the  positive  terminal  of  the  battery  and  the 
anode  of  the  cell.     The  pressure  of  the  hydrogen  gas  in  the  cell  was 
then  reduced  until  the  current  -flowing  between  anode  and  cathode  of 
the  cell  caused  a  faint  glow  to  fill  the  whole  tube.     The  metal  sur- 
face which  was  very  bright  before  the  illumination  appeared  in  the 
tube  afterv/ard  became  colored,  being  brownish  for  sodium,  bluish- 
violet  for  potassium,  and  light  greenish  for  rubidium  and  caesium. 
These  colors  are  due  to  the  formation  of  a  compound  of  the  hydrogen 
and  the  metal  which  is  called  a  hydride  or  an  alkali-hydride.  They 
found  that  after  this  forming  process  the  cells  were  three  to  four 
times  more  sensitive  than  v/ith  the  pure  metals. 

In  1211,  Elster  and  Oeitel-^  investigated  the  stability  of  the 
sensitiveness  of  photo-electric  cells  with  formed  potassium  cathodes 
and  hydrogen.     They  found  that  the  hydride  formed  on  the  surface  of 
the  metal  was  very  unstable  in  an  atmosphere  of  hydrogen,  and  further- 
more, that  after  about  two  months  a  decrease  of  about  85/o  in  the 
sensibility  was  observed. 

Ey  forming  the  hydride  surface  and  then  replacing  the  hydrogen 
by  one  of  the  inert  gases  argon  or  helium  the  cells  were  found  to  be 
more  permanent.     It  was  found  that  wlien  hydrogen  was  left  in  the  tube 
after  the  hydride  surface  was  formed  the  hydride  coloring  disappeared 
and  the  hydrogen  was  partly  absorbed  by  the  metal,     Tnen  argon  or 
helium  gas  was  placed  in  the  tube  a-^ter  the  metal  surface  was  formed 
the  colors  kept  quite  the  same  while  the  sensibility  remained  prac- 
tically constant, 

jl.  Phys.Zeitschr  .  .  Aug...  iCll.  


No  quantitative  itiea8ureni'=»ii  ba  hnv/^v^r,  hnvn  V.p'^n  ir-arle  for  de-  i 
termining  the  conditioijs  for  niaxinium  sen^i ibili t y  of  plio to-cl ec  tri o 
cells  of  alkali  n.etals  v/ibh  hydrogen  gas;  while  Elster  and  Oeitel's 
work  hao  been  iLore  of  a  qualitative  nature  yt  the  renults  are  very 
definite  in  fixing  the  g-^neral  facts  for  this  type  of  photo-electric 
cells. 

Turpose, 

The  puri'ose  of  this  in^^es tigation  is  to  make  a  systernatio, 
quantitative  study  of  the  conditions  of  sensibility  of  photo-electric 
cells  of  alkali  metals  with  hydrogen. 

This  investigation  necessarily  involves  the  study  of  the  effect 
of  variations  of  the  pressure  of  the  gas,  distance  between  electrQdes, 
area  of  metal  illuminated,  voltage  applied  to  the  electrodes,  and 
the  intensity  of  illumination. 

When  light  is  incident  upon  the  metal,  which  is  the  cathode  of 
the  cell,  and  a  potential  difference  is  acting  between  the  electrodes 
then  electrons  are  emitted  from  the  metal  at  the  cathode  causing 
ionization  of  the  gas  contained  in  the  cell.     Therefore,   some  remarks 
concerning  ionization  will  be  made  in  order  to  make  clear  v;hy  the 
study  was  made  mainly  along  the  lines  of  ionization. 

If  a  potential  differ-^nce  be  maintained  between  two  electrodes 
which  are  surrounded  by  a  gas,  a  current  will  flow  if  some  ionizing 
agent  is  caused  to  act  upon  the  gas.     The  ionizing  agent  may  be 
Roentgen  rays,  radioactive  substances,  ultra-violet  light  or- ordinary 
white  or  monochromatic  light  acting  upon  one  of  the  electrodes.  If 
the  current  flowing  between  the  electrodes  be  plotted  as  ordinates, 

and  the  potential  .iifferences  between  them  be  plotted  as  abscissae, 
then  for  a  constant  gas  pressure  a  curve  as  shown  in  Fig.  3  


7 

below  will  result.     For  email  potenti-.tl  Ji  T<?rpnce8  tli-^  curve  shov/e 


Fin.  2. 


that  the  current  varies  directly  as  the  potential  difference  applied, 
or  that  the  relation  ie  that  of  Ohm's  law.     At  the  point  a,   th>?re  is 
a  deviation  from  this  law  until  the  curve  becoiries  parallel  to  the 
F.D.  axis.     This  is  called  the  condition  of  saturation;  i.e.,  no 
increase  of  current  for  increase  of  potential  difference.     The  satur- 
ation current  ie  proportional  to  the  distance  between  the  electrodes 

point 

for  constant  pressures  of  the  gas.     At  the^b  the  curve  begins  to  rise 
showing  an  increase  of  curr'^nt  with  the  increase  of  rjotential  dif- 
ference.    This  is  a  very  important  point  on  the  curve  since  it  in- 
dicates the  minimum  pot'-^ntial  dif-ference  required  to  cause  sufficient 
potential  gradient  to  give  the  negative  ions  velocities  high  enough 
to  prod-:ce  other  ions  by  collision.     This  phenomenon  indicates  that 
to  I'roduce  an  ion  a  certain  minimum  amount  of  energy  is  required.  If 
this  critical  potential  gradient  be  known,  s-^y  E,   the  charge  6  of  a 
negative  ion,  and  its  mean  free  path  1,   then  this  minimum  energy  is 
W  =  Eel.     For  conditions  represented  by  the  curve  beyond  the  point 
b  the  positive  ions  reach  velocities  sufficiently  high  to  produce 
ions  by  c  'llision.     At  the  point  d  the  curve  begins  to  approach 
parallelism  to  the  axis  of  ordinates.     This  indicates  an  exceedingly 


8 

lar^e  increase  o"^  current  for  an  extreirely  aniall  increaBe  in  poten- 
tial differeiice  indicating  th  \     the  sparking  potential  has  been 

1 

reached.     Since  the  iii'tss  of  a  negative  ion  is  about    18C0  mass 
of  a  positive  ion  it  is  easily  seen  thnt  negative  ions  reach  the 
critical  ionizing  velocity  un^ler   the  action  of  a  much  smaller  poten- 
tial gradient  than  that  required  by  the  positive  ion. 

In  this  work,   therefore,   it  is  seen  th.^t  the  conditions  of  the 
problem  are  such  thit  t^e  study  of  the  effect  due  to  varying  the 
pressure  of  the  gas,   the  distance  between  the  electrodes,  and  the 
potential  difference  applied  to  the  electrodes  can  best  be  effected 
by  making  use  of  ionization  curves,     I^'Ioreover,   it  is  known  from  the 
theory  of  ionization  that  this  work  will  have  to  do  with  the  r^art  of 
the  ionization  curve  between  a  and  d. 

To  recapitulate;   the  study  is  made  by  varying  the  pressure  of 
the  gas,  P,   the  distance  between  the  electrodes,  D,   the  potential 
difference  between  the  electrodes,        the  intensity  of  illumination, 
L,  the  area  of  metal  illuminat  ed.  A,  and  reading  the  current  in 
terms  of  the  deflection,  d,  of  a  d'Arsonval  galvanometer. 


Detjcripbion  of  Apparatus, 

Figure  5i8  a  full  si^.e  drawing  of  the  details  of  the  glasa  tubes 
used  in  making  the  photo-electric  cells.     A  spherical  bulb,  3,5  cm. 
in  diameter,  has  two  tubes  1.0  cm.   in  diameter,   sealed  horizontally 
and  diametrically  opposite  each  other.     A  vertical  tube,   1.8  cm.  in 
diaraet'^r,  about  15  cm.  long  is  sealed  in  the  top  of  the  bulb. 

At  the  top  of  this  vertical  ^ube  a  platinum  '^rire  is  .-sealed  and 
fused  to  an  aluminium  rod  0.4  cm.  diameter  and  10  cm.  in  Isngtli,  To 
tlie  lower  end  of  the  aluminium  rod  is  attached  a  brass  spiral  spring 
to  which  is  connected  the  platinum  wire  anode.     The  anode  a,  being 
sealed  through  the  lower  end  of  the  gl  ass  t  ub  e  which  telescopes  the 
aluminium  rod.     At  the  ur.per  end  of  this  glass  tube  is  attached  an 
iron  ring  which  fits  neatly  inside  the  larger  tube.     By  means  of  an 
electromagnet,  using  a  current  of  2  amperes,   the  inner  tube  carrying 
the  anode  a,  can  be  held  in  any  desired  position  relative  to  the 
cathode  c,  at  the  bottom  of  the  l  ulb. 

At  the  bottom  of  the  bulb  and  diametrically  opposite  the  anode, 
a,  is  sealed  the  cathode,  c,   the  upper  point  of  which  does  not  extend 
beyond  the  surface  of  the  inside  of  the  bulb.     In  some  of  the  cells 
the  inside  of  the  lower  surface  of  the  bulb  was  silvered,   the  metal 
distilled  into  it  and  deposited  upon  the  mirror  surface.     In  this  way 
a  good  contact  was  insured  between  the  pl-^tinum  and  the  metal.  In 
some  of  the  tubes  the  metal  was  not  disti'^led  into  the  bulb  but 
poured  into  it  while  in  the  molten  state  and  allowed  to  solidify  over 
the  platinum  electrode.     The  metal  in  all  cases  was  used  as  the 
cathode  of  the  cell.     A  beam  of  light  could  be  passed  through  the 
walls  of  the  bulb  and  thus  be  incident  upon  the  active  metal  directly 
under  the  anode. 


FIG.  4. 


f 


13 

Tig.  4     i8  a  diap;ram  giiowing  th"  )ri<»Mio-l  of  oh.-^nging  the  diBtance 
between  the  elecToJes  by  means  of  tlis  elec  tronjfignet ,     The  electro- 
magnet   M  ,   ia  about  13  cm.   mean  diameter  with  its  axis  coinciding 
wit}i  the  axis  of  the  brass  scr-^w  f>,  v/iiich  has  a  pitch  of  one  milli- 
meter.    M  ia  attached  to  a  nut  v\hich  is  threaded  to  fit  the  screw  S, 
and  it  can  be  raised  or  lowered  by  turning  the  knob  B.     The  brass 
frame  ia  secured  to  th^  side  of  the  light-tight  box  shown  in  Fig.   5  . 
It  required  two  amperes  to  move  and  to  hold  the  electrode  a,   in  the 
desired  positions. 

The  distance  between  the  el^^c trodes  was  determined  as  follows:- 
Py  means  of  a  cathe tome tor  focused  on  the  upper  edge  of  the  telescop- 
ing tube  the  position  of  the  electrode  was  determined..    The  electrodei 
were  connected  in  series  with  a  dry  battery  and  a  telephone  receiver, 
which  gave  a  click  when  the  anode  and  cathode  were  in  contact.  The 
cathetoraeter  reading  for  the  position  when  the  electrodes  were  in 
contact  was  called  the  zero  reading;  and  the  diff'^rence  bet'^een  this 
reading  and  th'^t  of  any  other  position  of  the  anode  a,   gives  the  dis- 
tance between  the  electrodes. 

Fig.   5    shows  the  entire  arrangement  of  the  apparatus  for  the  in- 
vestigation.     The  photo-electric  cell  is  enclosed  in  a  light-tight 
box.     Wires  connected  to  the  anode  and  cathode,  and  to  the  electro- 
magnet pass  through  insulators  in  the  walls  of  the  box.     The  cell  is 
sealed  to  the  system  containing  a  "acleod  gauge  for  measuring  pres- 
sures up  to  0,24  cm.,  a  closed  manometer  for  measuring  the  higher 
pressures,  a  regulator  for  obtaining  small  variations  in  pressure,  a 
tube  containing  palladium  m'^tal  strips  for  supplying  pure  hydrogen 
gas,  and  an  air  pump.     The  palladium  metal  was  charged  with  hydrogen 
gas  by  the  electrolytic  method.     With  an  electrolyte  of  one  part  of 
HgSO^    and  three  parts  of  HgO,  the  anode  being  platinum,  and  the 


13 

palladium  metal   the  cathode,  hy>lrogen  gas  was  abeorb^^d  by  the  cathode 
when  C.5  volts  was  connected  across  the      ec  "rodes .      After  charging 
the  palladium,    the  tube  containing  it  whr  .sealr^d  to  the  syfitera.  When 
the  tube  is  heated  wi^h  a  small  bunsen  flame,   the  metal  '-jives  off 
pure  hydrogen  gas.     The  whole  glass  system,  when  the  pump  was  cut 
off,  could  be  filled  to  a  nreBsure  of  about  S5  era.  when  the  palladium 
was  heated. 

Small  variations  of  pressure  of  the  gas  could  be  produced  by 
raising  or  lowering  the  m'^rcMry  in  the  pressure  regulator.       The  elec-t 
tromajnet  was  connected  in  series  wi-^h  five  storage  cells,  requiring 
about  two  amperes  to  hold  the  anode  at  any  desired  position. 

The  galvanometer  used  is  a  Leeds  and  Tv^orthrup  type  HS,   ,  the 
sensibility  being  3.78  x  10"^*^  amp. per  mm.  deflection  for  3  meters 
scale  distance.     The  anode  of  the  cell  was  connected  to  the  earth 
through  the  galvanometer  and  a  megohm  resistance.     The  cathode  of  the 
cell  was  connected  to  a  variable  point  in  a  water  rheostat  which  is 
in  series  v/i  th  about  840  volts  from  a  storage  battery.     The  voltage 
ay-plied  to  cathode  could  be  ''■aried  by  means  o"^  the  water  rheostat  and 
it  was  measured  by  a  Kelvin  electrostatic  voltmeter  reading  0-600 
■'^olts. 

The  variation  o^  the  area  o^  the  metal  illuminated  was  obtained 
by  varying  the  opening  o^  the  iris  diaphram  A,  which  is  placed  at  the 
lower  end  of  a  brass  tube.     This  tube  was  blackened  on  the  inside  to 
prevent  reflection  of  light. 

The  intensity  of  illumination  was  varied  by  changing  the  posi- 
tion of  the  source  of  light  L  on  the  guide.     The  source  of  light  was 
a  4  candle  power,  110  volt  incandescent  lamp,   the  entire  bulb  of 
which  was  frosted.     The  candle  power  0^  hhe  lamp  after  frosting  was 
2.47  at  107  volts.     The  current  was  supplied  by  a  110  volt  storage 


14 


Fin.  5. 


15 

Py  mv-)^^ino;  th=  lamp  or.  the  guide  the  intensity  of  illumination  on 
the  metal  in  the  pho  lo-elec  trio  cell  could  be  ^railed,  and  this  varia- 
tion calculated  directly  by  means  of  the  inverse  square  law. 

Method. 

Hince  the  moat  8-'=n3itive  conUtions  for  the  photo-electric  effect 
are  being  sought,   it  is  necessary  to  study  the  effect  due  to  varying 
all  the  poasible  conditions  in  order  to  find  the  most  effective  set  of 
conditions.     The  variables  in  this  work  are  the  following:-  P  the  pres 
sure  of  the  gas,  D  the  distance  between  the  electrodes,  V  the  poten- 
tial difference  applied  to  electrodes,  A  the  area  of  metal  illuminated 
L  the  intensity  of  illumination,   t  the  temperature  o^  the  cell,  and  d 
the  galvanometer  deflection  which  is  proportional   to  the  current. 

Two  sets  of  readings  are  possible  for  each  cell,  namely:-  before 
forming  and  after  forming  the  hydride  surface.     In  this  paper  this 
process  will  be  called  forming. 

Four  cells  -  ere  studied:  one  v;i  th  caesium  and  three  with  potas- 
sium metal.     The  readings  were  taken  in  the  order  as  follows :-  with 
t,        A  J  P  and  D  constant  the  values  of  tlie  deflections  d,  of  the 
galvanometer  were  read  for  increasing  values  of  Thus  values  of 

current  and  voltage  were  obtained  for  an  ionization  curve.     This  was 
repeated  for  three  and  in  some  cases  four  distances  of  D. 

From  the  above  data  four  ionization  curves  are  obtained  which 
show  the  e-^'fect  of  varying  the  distance  betv;een  the  electrodes  for 
constant  values  of  t,  L,  A,  and  P.     If  the  above  three  or  four  ioni- 
zation curves  be  called  a  set,   then  it  is  possible  to  get  as  many 
sets  as  there  are  values  of  P,   the  gas  pressure.     From  three  tv-  five 


different  values  of  P  were  aelocted  for  e.ich  cell  and  in  this  way  Ihe 
effect  due  to  change  of  r,rea>3ure  was  e  tallied. 

Ionization  curves  were  also  obtained  in  '-rhich  A,  L,  P  ani  D  are 
constant  for   Mrree  and  four  different  t empera-^ur ea ,     Aft-^r  the  form- 
ing process  similar  sets  of  ionization  curves  were  taken  except  those 
for  temrerature  changes.     In  adiUtion  to  the  ionization  corves  taken 
after  forming  the  cell  !:o.  4,  sets  of  data  were   taken  in  which  h,  V, 
P,   t  and  D  are  constant  while  A  and  the  current  varied.     Also  reading 
were  taken  for  A,  7,  P,   t  and  D  constant  while  L  and  the  current 
var  i  ed . 


t)ata  and  Curves 

A  list  of  the  tables  of  data  and  curves  is  given  below, 
A  in  cm*^.  represents  the  area  of  metal  illuminated, 
L  in  candle  feet  represents  the  intensity  of  illumination, 
t  in  °C  the  temperature  of  the  cell. 

P  in  ram,  n-.ercury  represents  the  preaoure  of  the  hydrogen  gas, 
D  in  cm,  represents  the  distance  between  the  electrodes, 
V  in  volts  represents  the  potential  difference  between  electrodes, 
d  in  mm.  represents  the  galvanometer  deflections 


•J 


17 


Cell  No.  1. 
Caesium  IJetal. 

The  curves  are  all  ionization  curves, 
I,   8how8  effec^  of  variation  of  D,  before  and  after  forming, 
r  =  1.0  mm.  A  =  4.35  cm^.  L  =  0.186  C.f.   t  =  17^0 . 

IT,  8how3  effect  of  variation  of  ?,  be-^ore  and  after  forming 
D  =  0.5  cm.  A  ^  4.35  cm^.  L  =  0.186  C.f.   t  =  17oC. 

Ill,   shows  effect  of  variation  of  t,  after  forming,  A  - 
4.35  cm2.  L  =  0.186  C.f.  D  =  0.5  cm.  P  =  1.0  mm. 


21 

Plate  I.     Tlie  lower  curvse  wers  obtairi«»(l  te^oie  tli^  cell  was  formed 
while  the  uoper  cm"'^-.  were  obtained  '"vrter  rorrniiig; ,  The 
oorr eBporiding  curves  for  before  and  a-^ter  forming  t^how 
that  the  negative  ions  have  velocities  high  enough  to  cause 
ionization  by  collision  n  t  -ibout  the  same  "oltage.  "he 
voltage  required  for  illumination  is  much  Isss  before 
forming, 

Plate  IT.  All  th^.t  is  said  above  Tor  plate  I  is  true  for  this  plate. 

The  voltage  reqiured  for  illumination  is  much:  greater  for 
the  pressure  cf  l,C  mm.   than  for  the  other  curves.  These 
curv<*3  seem  to  show  that  ther'^  is      niinirnim  il  lurriinating 
voltage  for  a  certain  critical  pressure,     "^he  '^urvt  for 
1.0  mm.  pressure  show  the  best  photo-electric  condition 
for  sensitiveness, 

Plate  III. The  curves  for  different  temperatures  indicate  that  the 
voltage  at  which  negative  ions  attain  ionizing  velocity 
is  least  in  those  for  higher  temperature.  The  illumin- 
ating voltage  is  much  less  for  higher  teii^eratures  than 

for  higher  t'^^mper atures 
for  the  lower.     The  curves  show  conditions  more  sensitive 

to  photo-electric  effect. 


P.2 


Cell  No.  3. 
Potaasium  Metcal. 

The  curves  me  all  ionization  curves, 

Plate, IV,  shows  effect  of  variation  of  D.  A  =  3.17  cm^.  L  =  0.22  C.f, 
t  =  36°C.  P  =  5.5  mm.     Cell  not  formed. 

Plate,  V,  shows  effect  of  variation  of  D,  A  =  3.17  crr.'^.  L  =  0.22  C,f 
t  =  25°C,  P  =  4.0  mm.     Cell  not  formed. 

Plate, YI,   sh07/8  effect  of  variation  of  D.  A  =  2,17  cm^.  L  =  C.22  C.f 
t      24°C.  P  =  1.8  ram.     Cell  not  formed. 
Also  for  P  =  0.84.     t  =  24<^C. 


Plate, VII, shows  effect  of  variation  of  P. 

r  =  1,0  cm.     Cell  not  formed. 


A  =  2.17  cmS.  1,  ~ 


0.22  C.f 


?4 


Plate  IV.     These  curves  show  thn  for  this  pressure  the  order  in 

o  c  (J  ur 

vfhich  the  voltages  for  il lumination^i  a  the  saine  ae  that 
for  the  values  of  D, 
Plate  V,      These  curves  ohow  the  same  results  as  do  those  curves 
in  plate  TV. 

Pl-^te  VI.     The  curves  for  P  =  1,8  nun.  show  the  same  results  as  those 
in  Plate  IV.     The  curves  for  P  =  0.84  mm.   th'^  order  of  the 
curves  is  reversed  indi Gating  th'^  t  the  pressure  is  less 
than  the  critical  pressure  for  minimum  illuminating  volt- 
age.    The  curve  for  D  =  1,0  cm,  is  practically  the  same 
as  that  for  C  =  CO  cm,  and  D  =  3,0  cm,  and  for  this 
reason  was  not  plotted, 

Pl^te  VII,  These  curves  indicate  that  ':h9  critical  pressure  for  the 
minimum  i  luminat ing  voltage  is  in  the  neighborhood  of 
1.8  mm.    All  of  the  curves  in  the  plates  IV,        VI,  and 
VII  do  not  indicate  very  good  conditions  of  sensitive- 
ness since  the  ordinates  are  not  very  large  until  the 
illuminating  voltage  is  a-proached. 


28 


Cell  No.  3. 
Potassium  Metal. 

The  curves  are  all  ionization  curves. 

Plate,  VIII,  shows  the  e'^fect  of  variation  of  D,  before  and  after 
forming.     A      0.17  cm^.     L  =  0.22  C.  f .   t  ~  26° C. 
P  =  1,0  ram. 

Plate,  IX,     shows  the  e-^fect  of  variation  of  P,  before  and  after 
forming,     A  ~  2.17  cm^.  h  =  0.22  C.f,     t  -  26°C. 
D  =■  0.5  cm. 


70 


II 


5! 


So 


tf5  5: 


5: 


mm 


S5±r 


— o- 


m 


5: 


o 


.t±t 


"So 


Si 


U-  OF  P.  S.  S.  FORM 


31 


Plate  VIII.   The  curves  marked  A,   taken  after  forming,   show  thn t  the 

illuminating  ^''oltnge  is  in  the  order  of  their  values  for  D. 
The  curves  marked  B,   taken  before  ■forming,  ahoT^:  that  the 
illuminating  voltages  are  not  in  the  saiije  order  as  those 
marked  A,  and  that  curve  for  D  =  0.5  cm.   shows  an  illum- 
inating voltage  much  greater  than  that  for  D  -  l.C  cm. 
and  D  =  S.O  cm. 

These  curves  do  not  show  very  p:,ood  conditions  for  sensi- 
tiveness since  the  ordinates  are  very  small  until  illum- 
inating voltage  is  reached. 
Plate  IX.     The  curves  marked  A  show  that  the  critical  pressures 

for  minimum  illuminating  potential  is  m.uch  greater  than 
that  for  3.0  ma. 


CpII  No.  4. 

PotasRium  Metal. 

Data  and  Ionization  curvea  before  forming. 

Plate 

X     shows  pffect  of  var i'ition  of  D.  A  —  4.36  cm^.  L  = 

D .  22  C.f. 

t  =  24°G     P  =  60  rnrji 

Plate, 

XI     ahowfl  effect  of  variation  of  13 .  A  =  4.36  cm^ .  L  = 

0.22  C. 

f 

t  =  o4°G    P  =  5  0  mm. 

Plate 

XII     shows  e''^f'=ct  of  variation  of  P        A  =  4.36  cm^. 

L  =  0  23  C  f      t  =  25*^0      P  -  3.0  mm. 

Plate , 

XIII     shows  eff«=>Gt  of  variation  of  B.       A  =  4.36  cm^. 

L=0  22Cf       t-  26° C      P  =  2.0  mm. 

Plate, 

XIV     shows  off-^^nf  o-f  variation  of  D         A  =  4-36  cm^. 

Ii  =  0.22  C  f       t  =  26° C      P  =  1  0  mm 

Plate, 

XV     shows  effect  of  variation  of  D    A  =  4.36  cm^.  L  = 

0.22  C . 

f 

t  =  20° 0      P  =  3  0  rrm 

Plate 

XVI     shows  eff'='ot  of  variation  of  D    A  =  4.36  cm^.   L  = 

0 . 22  C • 

f. 

t  =  0°C      P  =  3-0  mm 

Plate, 

XVII     shows  eff'=ct  of  V'^riation  o"^  P        A  =  4  36  cm^. 

L  =  0.22  C.f.     t  =  25° G      P  =  3.0  mm. 

Plate, 

XVIII     shows  effect  of  variation  of  D.       A  =  4.36  cm^ 

J\  'I    ^  JL  ^  ^         9^  XX  Vy  tV  V             X   X  Vv  Vy    w       \y<^          V  LX  X  X  LjL  >J  X  yj  AX       W  X        JLy  9                    'A                    ^  ^  *^  V  V> 

L  =  0.22  C.f.     t  =  36°C      P  =  3  0  mm 

Plate, 

XIX     shows  effect  of  variation  of  P    A  =^  4.36  cm*^.  Ii 

y  \  X  y \  J       u  X  X  vy  T>  u      ^  x  x  \^  ^y  w     ^y  X      .*  la  j»  x  la  u  x  \y  x  x     \y  x            «                         *  4       \^      w  lii  • 

=  0.22c.  f. 

D  =■  1.0  cm. 

Ak/                 X  %  w       Vy  ii  i  9 

Plate, 

XX     shows  effect  of  variation  of  P    A  =  4  36  cm^     L  = 

0.22  C. 

f. 

T)  =  0-5  cm 

Ay             w  4  «.y     w  i>ii  • 

Plate, 

XXI.   shows  '=»ffeGt  of  formine^  rnet.^l.  A  =  4.36  cm^.  L  = 

0.22  C.f 

D  =  0  5  cm.  P  -  3.0  mm              Also  'for 

A  -  4.36  cm2.  L=  0.22  C.f.     D=1.C  cm.     P  =  3.0 

iT.m  . 

Tahl  '^R 

X  \-K.  krf  X  O 

uaud.  rix  e  Jiven  ueio-.  x  or  px<i  tes  Ai  ,  Al  v  ,  a \/ i. ,  a  v  i  i  1  ^  a 

IX, XX. 

33 


Critical  ^'"oltage  and  Current. 

In  this  cell  it  was  found  that  when  the  voltage  was  applied,  it 
could  be  increased  to  a  certain  definite  maxirnun.  value  ■be:^ore  a  de- 
flection of   '-he  galvanometer  was  noticeable  when  the  light  was  net 
acting.     If  this  voltage  was  exceeded  by  an  amount  hardly  readable 
on  the  voltmeter  a  deflection  of  the  galvanometer  reoulted,  Fi;rther~ 
more,  if  the  light  was  permitted  to  act  and  the  voltage  applied, 
equal  to  the  maximum  value  determined  as  stated  above,  a  definite 
deflection  of  ""he  galvanometer  was  produced;  and,  when  the  light 
was  suddenly  turned, off  the  galv-ncmeter  deflection  always  became 
zero.     However,   if  this  maxinium  value  of  th'^  voltage  were  exceeded 
and  the  light  turned  o^f,   the  deflection  of  the  galvanometer  was 
decreased  but  never  became  zero. 

This  ^'■oltage  there'' -^re,   represents  the  maximum  v/hich  may  be 
applied  to  tho  cell,  in  this  particular  caoe,  and  at  the  same  time 
ha^^e  the  ionization  current  pro"\iced  only  by  the  action  of  light. 
This  value  of  the  -oltage  I  shall  call  the  critical  voltage,  and 
the  corresponding  current  the  critical  current  for  this  particular 
condition  of  the  photo-elec ':r ic  cell.     The  critical  voltage  and 
the  cri'-ical  current  taken  together  give  a  definite  criterion  for 
determining  th?  best  conditions  for  sensitiveness.    When  the  critical 
current  is  a  maximum  and  the  critical  ^'■oltage  is  a  minimum,   then  the 
most  sensitive  conditions  obtain.     Therefore,   the  critical  voltag-^ 
and  the  critical  current  are  given  in  the  data  and  shown  by  the 
vertical  lines  as  for  example  a  b  in  plate  XI. 

As  was  stated  before,  -.vhen  the  cell  is  in  the  dark  the  current 
which  flows  is  not  measurable  with  the  galvanometer  when  the  voltage 
applied  does  not  exceed  the  critical  voltage.     If  the  voltage  is 


34 

increased  beyorad  the  critical  value,  when  the  cell  is  in  the  dark, 
the  current  haa  a  very  sudden  increase  3o  thit  it  cannot  be  measured 
with  the  galvanometer.     This  indicates  that  the  voltage  at  which  the 
cell  v;ill  illuminate  xvhen  in  the  dark  is  not  much  larger  than  the 
critical  voltage. 


Po  Ldb tii uiu  Ceil 

^jO.  4. 

Before  Formint^. 

P  = 

=  5 , C  mm .     t  = 

240c. 

L  =  0.22 

C  f  A 

-  4.35 

''■qI  ta  i/e 

i  Ti  iiiii'i . 

V  in  voltn. 

n  =  0, 

J  C  11;  . 

^  -  Z 

,  C  c  J  li 

■n  ;" 

d 

V 

V 

u ,  b 

375 

0"-" 

0,  0 

4^0 

C ,  1? 

4  7  w' 

1.0 

316 

1  0 

l.u 

C  'J 

1  •  '-^^ 

504 

1.  5 

331 

1  5 

408 

1.5 

480 

1.5 

535 

S.  5 

346 

421 

3.0 

500 

2.5 

559 

4.  5 

355 

4  '^8 

4.  3 

510 

5.0 

580 

8.0 

361 

.J  . 

446 

8.0 

520 

10.0 

600 

11.  5 

564 

O  .  O 

Tt  iJVJ 

11.0 

524 

1 1  5 

605 

16. 0 

36  c 

11.8 

455 

14.0 

5^6 

610 

39.  0 

566 

16.0 

457 

17,5 

528 

17.0 

615 

43.0 

568 

21.5 

458 

22.0 

70.0 

620 

50.0 

36G 

28.0 

460 

26.0 

531 

625 

115,0 

370 

56.0 

461 

31.0 

532 

630 

off 

462 

112.0 

536 

^  26. i 

PotaaBiuhi  Cell  T'o.  4 

r.efore  Forming, 

=  1 . 0  mm .     t  ~ 

26°  C. 

L  =  0.23 

A  ^  4.35 

cm*" 

"Ci-i  tical 

It age 

n  1 J .  ]    C  Li  r 

f^Tit  .  d 

i  J)    Ki/li , 

V    i  1.    ''■Q  1  ti^  . 

P  C 

.5  cni . 

.  0  c  rn . 

n  ^  2 

.  0  orn 

D  =  3.0  cm. 

0 .  5 

C.  D 

(O  OL) 

0 .  D 

Icli) 

0 . 5 

tJ  {  o 

l.C 

303 

1.0 

294 

1.0 

304 

1.0 

340 

3.0 

34-3 

3.0 

335 

4.5 

363 

1,5 

363 

5 .  5 

3  "  C- 

5.5 

345 

2,5 

370 

3,0 

389 

IC.O 

35S 

13.0 

351 

12.0 

372 

5.0 

400 

14.0 

360 

20.0 

352 

21.0 

374 

8.5 

405 

15.0 

560 

34, 0 

353 

27. C 

375 

14.0 

- 

35.  5 

363 

48.0 

354 

45.0 

376 

18.0 

408 

43.0 

365 

135.0 

355 

85.0 

377 

22.0 

409 

120.0 

355 

off 

388 

35.0 

3  5.0 

410 
411 

1  7.0 

355 

349 

G.O 

366 

400 

Fotasciiuui  Cell  Tio.  4 

r-e'^ore  Forming. 

p 

=  3,0  mnri.     t  =  0°C. 

L  =  0.22 

b.  I , 

A  =  4.35 

c  hi^ . 

.'^Critical 

0  1  1 1  ■ 

1  fi     (,iO,  . 

^'   i     vol  t  !^  . 

r  -  0.5  cm. 

■n  —  1 

J'  -  i 

.       C  Hi . 

r     ■  . 
,  C'    .11  . 

Ti  — 

,  < '  i>  11- . 

U 

V 

0 

V 

a 

V 

-1 
U 

V 

i; .  D 

0 , 

•  •- 

T  r. 

J.  ,  L' 

"i.!  P. 

1  .  u 

i  .  U 

/  A  K 

T  A 

44o 

0.  ^ 

o  .  u 

IDA 

i .  o 

lO  ,  U 

f  4 

O  ,  'J 

-i .  U 

o  ,  O 

/IT!-" 
4:  OD 

•7  A 

AQ'7 

o .  o 

4.  b 

A  A  n 

443 

D  .  O 

c;ac 
Dv.Jb 

t  o ,  u 

Jo  f 

op  n 

ot^  O 

D.  0 

44b 

Q  A 

Dl  O 

X 1  o .  u 

'iD  ,  u 

b 

T  A  A 

/I  CIA 

i  1  ,  D 

1  "7  r. 

1  r  O  ,  L' 

i  b.  U 

T  D  A 

c;  "7  A 

■^A  A 

455 

o  O .  U 

28,  0 

37.  0 

34.  D 

540 

55,0 

4b0 

38.0 

541 

70.0 

461 

48,0 

545 

58,0 

545 

65.0 

550 

78.0 

551 

"'  10.  0 

^  .0 « ^ 

TABLE    NO.  XVIII. 


.  „  "ell   ;-o,  4 

refor*?  "or mi  11*3. 

1 

=  3.0  ram.     t  =  38^0. 

L  =  0.33 

f .     A  -  4.35 

ifi  1,1,  . 

in  Tol  ^■  , 

D  =  C 

.  5  0  in . 

n  =  1 

.  0  cm. 

n  =  2 

.  0   0  Id  . 

1}  =  3.0  cm. 

•  i 

u 

i 

o.c 

0.5 

334 

0.5 

363 

0,5 

579 

2.3 

318 

1.0 

345 

1.0 

401 

1,8 

435 

6.0 

331 

3,2 

370 

3.0 

423 

3. 0 

450 

10.5 

323 

6.1 

382 

7.5 

428 

5,0 

460 

15.0 

334 

11.0 

384 

10.0 

430 

8,5 

470 

40.0 

335 

off 

385 

13,5 

4  31 

16,0 

480 

off 

326 

IS  .  5 

432 

32.0 

485 

31.0 

433 

29  .0 

49  0 

34.0 

434 

41.0 

49  5 

39,0 

435 

56.0 

5C0 

436 

76.0 

505 

70.0 

437 

oC  .  C 

^  7.5 

3S2 

7.5 

383 

11.0 

431 

3,0  . 

439 

39 


APl.F    m.  ^ix. 


PotasGium  Cell 

t:o.  4. 

Be-^ore  Forming, 

D  = 

1,0  cm. 

t  =  24°C.     L  ■ 

=  0 . 32 

C.I,  A 

=  4.35  cm2. 

i?Crioical  voltage  a- 

'it.  d 

i  n  mm. 

V  in  volts. 

p  -  1 . 

0  mm . 

P  =  2. 

0  mm. 

P  =^  3,' 

J  mm , 

P  =  5 . 

0  mm. 

P  =  8.0  mm. 

a 

V 

d 

V 

d 

V 

Cl 

V 

U  •  2 

3  3.0 

G  .  7 

0,0 

'Z  T  A 

^34 

Ot.'  -± 

i.  0 

on  A 

2y  4 

1.0 

395 

1 .  6 

304 

1 ,0 

38  i 

i ,  U 

AAA 

o ,  U 

335 

<0  •  ± 

320 

2,8 

3o7 

1  ,  b 

408 

i,  0 

O  •  D 

345 

4,4 

331 

4,0 

0 

0  ,  d 

431 

<j ,  U 

i  o.  u 

3ol 

13,0 

■7  T  0 

338 

0,0 

0  r  0 

3,  0 

A  TCi 

438 

0  ,  J 

<jU  .  U 

r?  CT 

353 

or) 

28  .0 

340 

10,  6 

»3o4 

D  .  0 

446 

Oil 

04,  U 

T  1 

3o  0 

13d  ,  0 

341 

18 , 0 

TO  C 

38d 

6,  8 

450 

5,5 

C  0  A 

b30 

354 

3y ,  U 

11  .  8 

7,0 

^  0  c 

Deo 

1 .3  -v ,  U 

1  iz  c: 

35d 

0  II 

TOO 

lb ,  0 

10.0 

5  30 

31,5 

458 

13,0 

535 

(jO  .  0 

460 

20,0 

540 

55 . 0 

461 

29,0 

541 

off 

462 

37,0 

543 

42,0 

544 

73.0 

545 

off 

546 

'  9,0 

349 

-1  ■  ■  n 

338 

29.0 

28,0 

460 

TAFL'^    NO.  XX. 


Potassiuu  Cell 

-0.  4. 

Be  '"ore 

D  =  0 . 5  cm. 

t  =  25°C.  L 

=  0,23 

C.f .  A 

=4.35  cm2 

#Critical  voltage  ind  current,  d 

i  n  mm . 

V  in 

volts. 

p  =  1. 

0  miri. 

P  =  2,0  ram. 

-  3. 

0  mm . 

C  ;-,!!.. 

0  mm . 

0.  5 

3G5 

0.5 

252 

0.5 

245 

0,5 

275 

0,5 

323 

1.0 

308 

1.0 

269 

29 1 

1,0 

316 

1.0 

359 

3.0 

346 

1.5 

280 

5 

303 

1,5 

331 

1.5 

380 

5,5 

355 

3.5 

291 

6,5 

316 

2,5 

346 

2.0 

390 

10.0 

35S 

6,5 

300 

10,0 

319 

4,5 

355 

2.5 

400 

14.0 

360 

14.0 

302 

14,0 

321 

8.0 

361 

3.5 

410 

18.0 

361 

20,0 

303 

23.0 

322 

11.5 

364 

4.6 

415 

2  o ,  Jd 

362 

28.0 

304 

32. C 

324 

16.0 

365 

6.5 

420 

43.0 

363 

40,0 

305 

34.0 

325 

29,0 

366 

9.0 

424 

130.0 

365 

145.0 

306 

150.0 

326 

43.0 

368 

12.5 

427 

50,0 

369 

14.0 

428 

115.0 

370 

19,0 

430 

27.0 

433 

36.0 

433 

65.0 

435 

off 

436 

7,0 

355 

26. C 

324 

28.0 

^7 


"<3 


sr. 

Pl.ite  X.     The  curves  show  thit  the  i llurnlnat inf^  voltages  are  in  the 

order  of  the  values  for  D  which  should  be  expected  for  this 
pressure, 

Plate  XI.  The  same  may  be  said  of  these  curves,   for  their  illuminat- 
ing voltages.     The  critical  voltage  for  D  ^=  0.5  cm.  is  the 
smallest  while  the  critical  current  for  D  =  1.0  cm.   is  the 
largest,   therefore,  th-?  best  conditions  ^or  this  pressure 
is  for  some  value  of  Ii  between  1  and  2  cm, 

Plate  XII.  The  illuminating  voltages  are  in  the  order  of  the  values 

of  B.     The  critical  voltage  for  D  =  0.5  cm,   is  the  smallest 
and  the  critical  current  is  the  largest,   therefore,  this 
curve  shows  best  conditions  for  sensitiveness. 

Plate  XIII.  The  order  of  the  illuminating  "oltages  is  the  same  as  thet 
for  values  of  D.     The  critical  voltage  for  D  =  0,5  cm.  io 
the  smallest  and  the  critical  current  is  the  largest,  there- 
fore, the  best  conditions  for  sensitiveness  are  shown  by 
this  curve. 

Plate  XIV.  The  order  of  the  illuminating  voltages  is  not  the  same  as 
that  for  values  of  D.     The  curve  f or  D  =  0,5  cm.   lies  be- 
tween curves  for  D  =  1,0  cm.  and  D  =  2.0  cm.     The  curve 
D  =  1.0  cm.  shows  best  conditions  for  sensitiveness. 

Plate  XV.  The  order  of  the  illuminating  voltages  is  regular.     The  con- 
ditions o"^  3<=»n8i  tiveness  are  much  better  as  represented  in 
curve  for  C  =^  3.0  cm, 

Plate  XVI,  The  order  of  the  illuminating  voltages  is  regular.  The 
best  conditions  of  sensitiveness  are  represented  by  a 
curve  thit  would  lie  betvveen  curves  for  L  =  0,5  cm  and 
C  =  1.0  cm. 


Plate  XVII.  The  otAqt  cf  the  illuminating  voltages  is  regular.  The 
best  conditions  ''or  sensitiveness  are  represented  in  the 
curve  for  D  =  0,5  era, 

Plate  XVIII.  The  order  of  the  illuminating  "':lt-i2es  is  regular.  The 
best  conditions  for  sensitiveness  are  represented  by  a 
curve  which  will  lie  between  curves  for  D  =  1.0  cm.  and 
D  =  2.0  cm, 

Plate  XIX.  The  curve  for  P  =  1.0  mm.  lies  between  curves  P  =  2,0  im, 
and  P  =  3.0  mm.   showing  that  the  critical  pressure  for 
minimum  illuminating  voltage  is  in  the  region  of  2  mm.  The 
best  conditions  of  sensitiveness  are  represented  by  a  curve 
which  lies  between  curves  for  P  =  3,0  mm.  and  P  =  5.0  ram. 

Plate  XX.  The  curve  for  P  =  1,0  mm.  lies  between  curves  for 

P  =  3.0  mm,  and  P  =  5,0  im.     This  indicates  that  the  crit- 
ical pressure  at  ivhich  the  illuminating  voltage  is  a  mini- 
mum lies  in  the  region  of  P  =  2,0  mm.     The  best  conditions 
for  sensitiveness  are  represented  by  a  curve  which  lies 
between  curves  for  pressure  between  2  and  3  mm, 

Plate  XXI,  The  upper  curves  are  for  D  —  1,0  cm.  before  and  after 

forming.     They  show  th?>.t  the  illuminating  voltage  is  less 
for  the  curve  after  forming.     The  conditions  for  sensibility 
are  better  as  shown  in  curve  taken  after  forming.  The 
lower  curves  are  for  D  =  0.5  cm,  before  and  after  forming. 
The  same  as  above  may  be  said  of  these  curves. 


Cell  No.  4. 
Data  and  Ionization  curves  after  forming, 

Plate,  rKII,   shows  the  effect  of  ^'ari:ition  of  D  for  P  =  10  ram. 

A  =  4.3G  cm2.  L  ^  0.22  C.f.     t  ^  24°C. 
Plate,  XXIII,   shows  the  effect  of  variation  of  D  for  P  =  5.0  mm. 

A  =  4.36  cm2.  L  =  0.22  C.f,     t  =  250C. 
Plate,  XXIV,  shows  the  effect  of  variation  of  D  for  P  =  3,0  mm. 

A  =  4.36  cm2.   L  =  0.22  C.f.     t  =  25«C. 
Plate,  XXV,   shows  the  effect  of  variation  of  D  for  P  =  2.0  mm. 

A  =  4.36  cm2,  L  =  0,22  C.f.     t  =  26°C. 
Plate,  XXVI,  shows  the  «rrcct  of  v^.riation  of  D  for  P  =  1,0  ram, 

A  =  4.36  cm2.  L  =  0,22  C.f.     t  =  2S'^C, 
Plate,  XXVII,  shows  the  effect  of  varintion  of  P  -^or  D  =  1.0  cm, 

A  =  4.36  cm2.  L  =  0.22  C.f. 
Plate,  XXVIII,   shows  effect  of  variation  of  P  for  B  =  0.5  cm. 

A  =  4.36  cm2.  L  =  0.23  C.f. 

Tables  of  data  are  given  below  for  the  plates  XXIII,  XXVI,  XXVII, 
and  XXVIII, 


1  :Iq,   -t.  AfLe 

L'     1  O  i'llii 

=  5.0  Rirn.     t  = 

25"  0. 

L  =  0.22 

G,  f. 

A  =  4.35 

cra^ . 

for  i  1 1 a  1 

vol  t  "^.j-e 

'iTjii  owl 

V  i  n  vcj  1 1 H  . 

■n    ^  ^ 

p  -  - 

«  •  >    >  ■  J .  , 

J 

•.i 

1 ,  c 

30  0 

1 .  r 

1.0 

.1 1 

1.5 

355 

4 .  5 

3  :i 

3 . 0 

■7  -7  O 

•  J  •  J  .  J 

7.4 

4  30 

2.5 

410 

11 . 0 

350 

5.0 

363 

17.0 

472 

7.5 

494 

30,0 

351 

11.0 

327 

29  , 5 

482 

10.  5 

508 

35.0 

367 

34.0 

417 

45.  5 

490 

17.  5 

534 

54,0 

370 

28.  5 

422 

70.0 

495 

28.  5 

550 

84.0 

372 

1 25 . 0 

430 

108.  0 

500 

41.0 

560 

13G.  0 

374 

175.0 

431 

188.0 

505 

57  .  5 

570 

370. 0 

37  5 

12  2.0 

433 

340.  0 

506 

72.0 

575 

80  .0 

o  SO 

107.0 

585 

128.0 

590 

148,0 

595 

179  .0 

600 

205.0 

605 

238.0 

610 

#  370. r 

'S'  ■   •  ^ 

'  1  .  . 

'1  .  - 

—  ^^ 

J  1  .  . 

Potaesium  Cell  No.  - 

4.       After  "onuing. 

V 

=  ]  .0  him.     t  =  36" C, 

L  =  0,22 

0 .  f . 

A  -  4.35 

c  m"^ .  ■ 

Vol  t  :\^Pt  .  J    \  c  ui  I  ?n  t ,  d 

'xl^    IIiliI  , 

V  i  n    ty  1  to  . 

D  =  0.5  cm. 

V  -  1 

.  0  cm. 

D  =  2 

,0  cm. 

D  =  3.0  cm. 

1 

V 

1.0 

i  '6  0 

1 . 0 

i '  C 

1 .  c 

153 

l.C 

o  7  r, 

<o  .O  vJ 

3.0 

220 

3.0 

239 

3.  5 

275 

3.5 

315 

7.5 

270 

5 . 0 

270 

7.5 

33  5 

10.5 

360 

13.5 

285 

10.0 

29  5 

11.5 

330 

19.0 

376 

?1.0 

295 

20 . 5 

310 

16  .5 

340 

29  . 0 

385 

37.0 

300 

41 .  0 

330 

31 . 0 

350 

41.0 

39  0 

55.0 

57.0 

3.32 

45.0 

355 

62.0 

395 

305 

83,  0 

325 

66.0 

T  O 
tJ.JO 

81.0 

397 

102.0 

306 

114.0 

326 

88.  0 

360 

L  .  U 

398 

117.0 

307 

327 

119  .0 

361 

112.0 

400 

145.0 

308 

1G8.0 

328 

137.0 

362 

155.0 

401 

158.0 

309 

19  3.0 

329 

154,0 

^ 

187.0 

402 

off 

310 

180.0 

364 

o  65 

218,0 

off 

403 

'*  i>  r 

^  55.0 

303 

351 

TO  O 

Potci3siura  Coll 

Mo.  4. 

Ti  = 

1.0  cm. 

t  =  2 

5°C.  L 

=  0,22 

C.f,  A 

=  4.35  cm2. 

■^Critical  vclt^.Pis  ini  curr'^ 

nt.  d 

i  n  Yi\m . 

V  in 

'      -    1  . 

0  jr. 111. 

?  -  2,0  in;:;. 

V  ^-  ::. 

0  ri.ir:. 

_ 

•              .J  • 

C  ,  .... 

d 

V 

1.0 

160 

1.0 

IS  I 

1.0 

221 

1.0 

243 

1.0 

360 

3,0 

239 

3. 5 

260 

8.0 

338 

3,0 

332 

3.0 

472 

5,0 

270 

7.0 

291 

13,0 

351 

5.0 

363 

5,5 

535 

10.0 

295 

11.0 

305 

21.0 

360 

11,0 

39  7 

10.0 

566 

20.5 

310 

27.0 

320 

32,0 

365 

24,0 

417 

16.5 

590 

41.0 

320 

49.0 

325 

57,0 

369 

28.5 

422 

19.5 

600 

57.0 

322 

71.0 

326 

64,0 

370 

125.0 

430 

26.5 

615 

83.0 

325 

85.0 

328 

113,0 

372 

175.0 

431 

30.5 

620 

114.0 

326 

140.0 

330 

200.0 

374 

192,0 

432 

38.0 

630 

133.0 

327 

192.0 

331 

280.0 

375 

168.0 

328 

off 

332 

193.0 

329 

off 

330 

73.0 

324 

190.0 

331 

•-I  ,^  '7,  ^ 

774 

r 

TABLE    NO.  VXVIII. 


Potassium  Cell 

Ko.  4. 

Af  t- 

r  Forming. 

D  = 

0.5  cm. 

t  -  35<>C.  L 

=  0,22 

C.f ,  A 

=  4.35  cm^. 

#Criticnl  volt'igs  ind  current.  d 

in  ram. 

V  in 

^'■ol  ts. 

p  =  1, 

0  mm . 

V  - 

-  3, 

0  rcrn . 

P  =  5. 

0  rirn. 

P  =  10 

mm . 

d 

V 

:1 

'/ 

1.0 

150 

1.0 

165 

1.0 

19  6 

1.0 

200 

0.5 

350 

3.0 

320 

4.0 

250 

3.5 

265 

4.5 

319 

1,0 

312 

7.5 

370 

8.0 

371 

5.5 

289 

11.  C 

350 

2,0 

361 

13.5 

286 

13.0 

380 

10.0 

305 

20,0 

361 

3,0 

388 

21.0 

3S5 

31.0 

385 

16.0 

315 

35.0 

367 

5.5 

420 

37.0 

300 

31.0 

390 

22.0 

320 

54.0 

370 

9.0 

440 

55,0 

303 

55.0 

294 

32.0 

322 

84.0 

372 

12.0 

451 

70.0 

305 

.  91.0 

295 

54.0 

326 

136.0 

374 

21.5 

455 

102.0 

306 

125.0 

296 

71.0 

327 

270.0 

375 

30.0 

470 

117.0 

307 

140.0 

29  7 

90.0 

338 

38.0 

472 

145.0 

308 

194.0 

298 

115.0 

329 

62,0 

475 

168.0 

309 

off 

299 

185.0 

330 

73.0 

47  6 

off 

310 

off 

331 

99.0 

477 

155.0 

478 

280.0 

479 

55,0 

12  3.0 

0  ff 

331 

270.0 

375 

280.0 

4-^9 

ei 


Plate  XXII.  The  order  of  illumir,at ing  voltages  ia  ref^ular.     The  beot 
conditione  for  aensitivenefls  are  shown  by  curve  for  L  ~ 
0,5  cm, 

Plate  XXIII,  The  order  of  illuniinat ing  volt'^g'='s  is  regular.  The 

best  corditions  for  sensitiveness  are  shown  by  curve  for 
D  =  0,5  cm. 

Plate  XXIT.  The  order  of  illuminating  voltages  is  regulat.     The  best 
conditions  for  sensitiveness  are  shov;n  b  y  curve  for 
D  =  C.5  cm. 

Plate  XXV.  The  order  of  illuminating  voltages  is  regular.     The  best 
conditions  for  sensitiveness  are  shown  by  a  curve  y;hich 
lies  between  curves  for  D  =  0.5  cm  and  D  =  1,0  cm. 

Plate  XXVI.  The  order  of  illuminating  voltages  is  regular.     The  best 
conditions  for  sensitiveness  are  shown  b^''  a  curve  which 
lies  betweer  curves  for  D  =  0.5  cm  and  D  =^  1,0  cm. 

Plate  XXVII.  These  curves  show  conditions  of  sensitiveness  for  differ 
ent  pressures  and  D  =  1,0  cm.     The  curve  representing  best 
conditions  for  sensitiveness  lies  near  the  curve  for 
P  =  3,0  mm. 

Plate  XX^rill.   The  curves  show  the  conditions  of  sensitiveness  for 

different  pressures  and  D  =  0,5  cm.     The  curve  representing 
best  conditions  for  sensitiveness  lies  near  the  curve  for 
P  =  3,0  mm. 


Variation  of  Area  Illuminated. 
After  Forming. 

Plate,  XXIX,   shows  the  effect  of  variation  of  A  for  P  =  10  mm. 

D  =  0.5  cm.  V  =  570,  490,  475,  L  and  t  constant. 
Plate,  XXX,  shows  the  effect  of  variation  of  A  for  P  =  5.0  ram. 

D  =  0.5  cm.  V  =  366,  365,  364.  L  and  t  constant. 
Plate,  XXXI,   shows  the  effect  of  variation  of  A  for  P  =  3.0  mm, 

D  =  0.5  cm.  V  =  323,  331,  320.  L  and  t  constant. 
Plate,  XXXII,  shows  the  effect  of  variation  of  A  for  P      3.0  mrn. 

D  =  0.5  cm.  V  =  395,  394,  393,  L  and  t  constant. 
Tables  of  data  are  given  below  for  plates  '  XXXI  and  XXXII. 


P  =  3.0  mm.     D  ^  0,5  cm.     t  =  25"n.  Unit 
of  A       0.0R8  omS.     T,  =  0.22  H.f,     r]   i  n  >']rr,. 

A 

A 

A 

8 
18 
32 
50 
70 
105 
135 

1 

4 
8 
16 

,1 

■  J 

1 

18 

2? 

75 
120 

J. 
2 
4 
8 
16 
32 
64 

5 
11 
17 
30 
47 
63 

r> 

4 
8 
IG 
32 
64 

r 

0  I  'A  3     1  U!U 

Cell  ::o. 

.  4, 

Aft 

3T 

rorming 

_ 

1'  — 

2.0  mm. 

D  =  0.5  era. 

t 

2.^ 

Unit 

of 

A 

=  0.068              L  ^- 

^2 

.  f  . 

d 

v  -  <? 

-r  •  G  ;l 

TT 

d 

A 

d 

A 

d 

A 

5.5 

1 

27 

2 

8 .  o 

2 

7 

2 

32 

4 

iO 

4 

12 

4 

54 

8 

28 

8 

20 

8 

80 

16 

45 

16 

35 

16 

115 

32 

66 

32 

52. 

32 

140 

64 

83 

64 

63 

64 

71 


U.  OF  I.  E.        rOMM  B 


U-  O;    :.  S.  S.  FORM  3 


7-1 

Curves  3liowirig  Frfect  of  "^^ir iation  of  Ar'^.T  MRtal 

1 1. laminated* 
Plates  XXI.X,  XXX,  XXXI,  XXXII. 

The  plotted  points  Jo  not  fall  on  the  vgraooth  curves  in  plate 
XXIX  as  nicely  as  they  do  in  the  other  plat^a.  The  form  of  these 
curves  indicate  that  variations  in  small  areas  illuminated  produce 
large  chmges  in  the  curr^^nt,  and  variations  in  large  areas  illum- 
inated produce  small  changes  in  the  current.     These  facts  show  that 
there  is  a  aaximum  area  of  illumination  for  ""his  particular  type  of 
photo-electric  cell  v;hich,  if  exceeded  will  not  increase  the  sensi- 
tiveness. 

The  curve  should  be  a  straight  line  if  the  electric  field  were 
uniform,  the  surface  conditions  uniform,  with  no  reflection  of  the 
light,  and  no  shadow  caused  by  the  electrode.     In  this  cell  the  above 
conditions  were  not  fulfilled  therefore,  the  form  o""  the  curve  is  not 
a  straight  line. 

Variation  of  Intensity  of  Illumin-^ tion. 
After  Forming, 

Plate,  XXXIII,   shows  variation  of  current  with  intensity  of  illumina- 
tion for  P  =  10  jrjn.  D  =  1,0  cm.  V  =  570.     For  P  =  10  mm, 
D  =  0,5  cm.  V  =  490.     For  P  =  10  mm.  15  =  0.5  cm.  V  =  475. 

Plate,  XXXIV,   shews  variation  of  current  with  intensity  of  illumina- 
tion for  P  =  5  mm.  D  =  0.5  cm.  V  =  3G6,   365,  364. 

Plate,  XXXV,   shows  v?.riation  of  current  with  intenaity  of  illurainatior 
for  P  -  3.0  mm,   D  =  0,5  cm.  V  =  332,   321,  320, 

Plate,  XXXVI,  shows  variation  of  current  with  intensity  of  illumina- 
tion  for  P  =  2,0  mm.  D  =  0.5  cm.  V  =  295,  294,  293. 

Tables  of  data  are  ?iven  below  for  plates    XXXIV    and    XXXV  . 


?ota88ium  C 

ell  !Io. 

4 .       A  rter  Forniing, 

P  -  5  .  0  miii . 

D  0.5 

cm.     t  = 

pro  n 

Un  .1  t 

of  L  - 

-  c.rc  X 

A  -  4.  35 

c  la*^ .  d 

i  i  1     II,  ■!.  . 

V  =  366  volts 

V  ^  365  volts 

-  364  vol  t9. 

d 

T 

0.11 

.  i 

0.1] 

10 

0.16 

n  .5 

0,16 

22 

0.16 

16 

0."5 

20.0 

0.25 

33 

0.25 

19 

0.  33 

27.0 

0.33 

44 

0.33 

25 

0.44 

35.0 

0.44 

60 

0.44 

37 

0 , 64 

53.0 

0.64 

83 

0,64  ■ 

60 

]  .00 

1  ,00 

T  30 

1  ,  OG 

To 

si Ltii'  Cell  i'o. 

■4.  Af. 

•3r  .  17  i.'  ml  . 

•n 

1 

3.0  mm . 

C  =  0.5 

cm .     t  = 

250c. 

TJni  t 

L 

=  0.22  X 

10-4. 

A  ^  4.35 

c  m*^ .  d 

•7  -7 

-     a.  u  t> 

I  ''•olts 

L 

d 

L 

.  d 

T 

O.ll 

0.11 

0 . 16 

17 

0.16 

9 

0.16 

0.25 

0,25 

14 

0.25 

0 .  33 

3 '7 

0.33 

19 

0.33 

0.44 

50 

0.44 

27 

0.44 

0.64 

70 

0.64 

40 

0.64 

131 

1.00 

115 

1.00 

.60 

1 ,  00 

U-  OF  I.  3.  S.  FORM  3 


U.  CF  1.  S.  S.  FOH^'  3 


80 

Curvee  showing  the  Effect  of  the  Variation  of 
Inteiiaity  of  Illumination, 
Plates  XXXII  to  XXXVI. 

The  points  for  the  smaller  voltages  lie  more  nearly  on  a  straighl 
line.     The  largest  error  in  the  intenaity  "^or  the  point  farthest  from 
the  line  is  7>  of  ,15  C,f,  or  ,01  C.f,     The  points  for  smaller  inten- 
sities show  much  smaller  deviations  than  those  for  larger  intensities. 
The  higher  voltages  applied  caused  unsteadiness  of  the  current,  hence, 
the  galvanometer  deflections  are  liable  to  larger  errors. 

The  most  sensitive  conditions  represented  py  a  curve  for  various 
plates  are  tahulat^d  in  the  form  below.     From  this  table  the  con- 
ditions of  highest  sensibility  may  easily  be  selected. 


Table  to  show  best  conditions  for  senei tivenesa, 
Y  =  critical  ^'■oltage. 

I  =  critical  current  in  galvanome f^r  deflections, 
r  =  best  "listance  between  electrodes  in  cms. 
P  =  best  pre3?.ure  in  tmr. 


■Pefore  "forming  n-Tefiil, 

I 

XI 

460 

to  527 

16 

to 

28 

1  to 

2 

5.0 

XII 

324 

32 

0.5 

3.0 

XIII 

304 

26 

0.5 

2.0 

XIV 

349 

to  366 

6 

to 

9 

1  to 

2 

1.0 

XV 

454 

8 

2 

3 

XVI 

335 

to  39  6 

10 

to 

45 

0.5  to 

1.0 

3 

XVI I 

324 

32 

0.5 

3 

XVIII 

383 

to  431 

7 

to 

11 

1  to 

2 

3 

XIX 

387 

to  460 

28 

to 

29 

1 

3  to 

5 

A  X 

304 

to  324 

26 

tc 

32 

2  to 

n 

After  Forming  Metal. 

n'll 

4  79 

2B0 

0.5 

IC 

XXIII 

375 

270 

0.5 

5 

XXIV 

331 

off  scale 

0,5 

3 

XXV 

§9  6 

to  331 

12 

3  tc 

.  ISO 

0,5  to 

1.0 

2 

XXVI 

303 

to  324 

55  tc 

72 

C.5  to 

1,0 

1 

XXVII 

374 

203 

1 

3 

XXVIII 

331 

off  scale 

0.5 

3 

Values  of  t. 

(1)  about  -20° C    salt  and  ice. 

(2)  QOC. 

(3)  25°C. 

(4)  38«C. 


82 

By  inspection  of  the  taMe  above,   it  is  seen  that  the  beat  con- 
ditions for  sensitiveness  before  forming  are  about, 

V  =  300  volts. 
D  =  0,5  era, 

P  =  2  to  3  mm. 
t  —  C 

And  the  best  conditions  for  Sf?n8i tiveness  after  forming  are  about, 

V  =  330  volts, 
D  =  0,5  cm, 

P  =  3  ram. 
t  -  25«C. 

The  cell  is  ibout  100  times  more  sensitive  after  forming  than  before 
forming. 


83 


Theoretical  rieduction  on  M3aGuremerit  of  Intensity 

of  Illumination. 

If  th«?  intoniUty  o^  i llurrr  n^.tion  varies  dir°ctly  with  the  cur- 
rent for  very  smnill  intensities,  then  it  it?  po^gible  to  calculate 
the  intensity  measvirable  with  in  instrument  of  given  sensibility. 
From  the  Fig,   6    belo;v  it  is  seen  th"t  for  the  curve  of  366 


s 


yir;.  6. 


130 


volts,  plate  XXXIT,   the  tan  ^  -  ~  -  2qo 
cd  =  I  current  flov/ing 
c  =  3.78  X  10~^^  amp, per  ram. 


=  0.65,  or  S 


tan  9  0,65 


d  = 


ram. 


0.65  X  3.78  X  10-10 
but  3  ~  ~j-2  ,  and  for  particular  values  of  S  =  200,.  and    ^  -  ( 100cm)? 

c  =  S/^  =  200  X  (lOO)*^cra.  =  2  x  10^  cm; 
106 


,  or,  /^J^:^ 


substitute  value  of  S  =  3,78  x  10-TU 


then. 


J  =        X  10^  x~d.65  X  3.78  x  To"^    =  /1. 9  "x  10-^ 


I  I 


By  means  of  an  electrometer  a  current  I  =  IQ-l'^  can  be  measured. 


84 

Substituting  this  value  of  I  in  the  eiuntion  i"^ove,  then 

X  =  v^-AAQ^^    =  2.21  X  lO'^  cm.  or  321  meters,   the  distance  the 
10-13 

2.47  c.p,  lamp  could  he  removed  from  the  cell  and  still  he  detected. 

By  means  of  a  tilted  electroscope  a  current  T  =  10"1^  amperes  can 
"be  measured. 

Substitute  this  value  in  the  equation, 

/  =  J?^  ^  IQ"^    =  74.9  X  IqH  =  7  X  10^  cm.  or  7  kilometers  or 
10-15 

4,3  miles. 

The  2.47  c.p,  lamp  could  be  detected  by  means  of  an  electroscope 
at  a  distance  of  4,3  miles.     To  detect  a  candle  instead  of  the  2.47 
c,p.  lamp  at  4,3  miles  distance  by  means  of  the  tilted  electroscope 
of  10""15  sensibility  the  distance  could  be  as  follows: 
Since  the  intensities  of  illumination  of  cell  must  be  the  same  then, 
3.47      _      1  /  _  ._4.3        _     ^  , 

Professor  Joel  ntebbins  in  his  work  on  measuring  the  variation 
of  intensity  of  illumination  of  the  variable  stars  Algol 'and  others 
used  a  selenium  cell.     It  is  possible  to  detect  a  candle  at  a  dis- 
tance of  500  meters,   or  0.3  miles,  V7ith  such  a  selenium  cell. 

The  equation  for  the  distance  at  which  this  postassium  cell  is 
sensitive  v,'hen  the  current  is  measured  with  a  tilted  electroscope  is, 
and  for  some  other  distance  it  is   ^    =  v/  — 


•2 


^    =  T y  =  7  X  10^  cm.  =  5  X  10^  cm, 

Iz,  ^^1 

^         49  X  10 1^  ? 

then    -jT-    =   =  2  X  10^  aDoroximately 

26  X  108 

Thus  it  is  seen  that  the  potassium  cell  is  a":out  two  hundred  times 
m.ore  sensitive  than  the  selenium  cell. 


Fr^r^jy  required  to  produce  an  Ion, 

TO  produce  an  ion  a  "ertain  minimum  amount  o-^  energy  is  required 
This  energy  is  thit  required  to  draw  an  electron  out  of  an  isolated 
molecule  against  the  force  of  attraction  of  the  positive  charge  of 
the  molecule.     In  Fig.  7     let  "R-^  be  the  ra.iiua  of  the  molecule,  , 
the  positive  charge,  G3  the  nega'-ivc  charge  of  electron.     If  the 


FIG.  7. 

electron  be  displaced  a  distance  dR  the  work  done  will  be 
dW  =  ~1 — 2_  dR,     The  total  energy  re:iuired  to  draw  an  electron  out- 


R 


2 


1 

side  of  the  influence  of  the  positive  charge  is 


00 


W    ^    /  ?L1A  dR  =  "^X-fe    unii-g  Qf  work. 

-^/f.   r2  Ri 

61=63  =  4.67  X  10~-0  y.,   S.  U. 

R^  =  10"*^  cm.  for  hydrogen. 

(1)    W  =      *       ^Q«8~"        ~  2,18  X  10*"^^  ergs,  the  minimum  amount  of 
energy  required  to  draw  an  electron  from  an  isolated  molecule  of 
hydrogen  or  to  produce  a  hydrogen  ion. 

For  an  isolated  molecule  it  is  necessary  to  withdraw  the'  elec- 
tron to  an  infinite  distance  from  the  center.     If  the  molecule  how- 
ever, is  in  an  electric  field  the  force  of  attraction  between  the 
molecule  ,?nd  the  electron  is  zero  at  a  distance  say        from  the  center 


86 

The  effect  of  the  electric  field  tends  to  decrease  the  energy  ic?- 
quired  to  withdraw  the  electron  on  one  aide  of  the  molecule  (with 
references  to  the  dirpotion  of  '^xt^rnal  electric  field)  while  that  on 
the  opposite  side  is  increased. 

Let  Fig.   8    represent  the  molecule  of  radius      ;  H  is  the  direc- 


FTG.  8. 

tion  of  the  external  electric  field  in  the  direction  of  motion,  Oa, 
of  the  electron.     Let        be  the  distance  beyond  which  the  electric 
attraction  between  the  positive  charge  of  the  molecule  and  the  elec- 
tron ie  zero.     The  work  required  to  withdraw  the  electron  beyond  the 
influence  of  the  molecule  is, 

W  =         /      ilJ2        ^  ^  gl  ^3  ^  gl  H  _  £lj2 

4      h2  R  Rl    -  Rs 

but.e^  =  ^2  =  5  ;  and  it  is  reasonable  to  assume  R3  =  4  , 

Rl     4RJ       R3L    ^  '   >  4 

Substituting  the  values  for  R^  and  S,  then 

fPl     TT  -  3  (4,67)^    X  IQ-^^Q  _  .  ^„ 

\d>)     A'  1-   =  1.63  X  10  ergs, 

4  X  10-8 

Thus  the  value  for  H  in  calculation  (2)  is  three-fourths  that  in 

calculation  (l).    Hence  2.1-8  x  lO^^^  ergs  is'  not  the  minimum  value. 

The  minimum  amount  of  energy  required  to  produce  an  ion  b  y 


collision  cnn  be  det^riuined  roughly  from  the  -In^ri  tf.V.on  in  this  in- 
vestig.ition.     For  the  conditions  of  this  work  t-hs  minimum  ener^^y  is 
Tf  =  E  C^,  where  T.  is  th;^  potential  gradient,  or  electric  force,  e 
is  the  charge  of  a  negative  ion  or  electron,  and  ^  is  the  mean  free 
path  of  an  ion.     The  force  E  can  be  determined  roughly  as  follows: - 
The  voltage  applied  to  the  electrodes  of  the  cell  necessary  to  pro- 
duce velocities  high  enough  to  cause  ionization  by  collision,  is  ob- 
tained from  the  ionization  curves.     This  voltage  is  shown  very. dis- 
tinctly at  the  point  on  the  curve  where  the  ordinate  or  current  shows 
an  increase  after  the  saturation  state  has  been  reached.     Let  V  be 
this  voltage  taken  from  the  curve,  and  let  D  be  the  distance  between 
the  electrodes.  Then, 

V 

^  "  300  D  ^*  ^' 

a)  First  assumption  regarding  mean  free  path  of  electrons. 

Assuming  that  the  negative  ionb  or  'Electrons  and  the  molecules 
of  the  hydrogen  gas  act  as  a  mixture  of  two  gases,  the  equation  of 
t  ^fi  mean  ^lee  path  given  by  Max;"ell  in  the  kinetic  theory  of  gases  is 


/=  : 


s 

/ 


^  m-j^ 


for  the  electrons. 


for  hydrogen  molecules. 


For  an  electron         the  diameter  of  the  sphere  of  action  when  two 
electrons  collide  is  practically  zero,     Eut    OJi  the  diameter  of  the 
sphere  of  action  when  an  electron  collides  with  a  molecule  of  hydro- 
gen is  assumed  equal  to  the  radius  of  the  molecule  or  10""^  cm, 

is  the  mass  of  an  electron  eq:;.al  to  8.8  x  10"''^°  grams. 
IvU  is  the  mass  of  a  hydrogen  molecule  equal  to  1,6  x  10"^'^  grams. 

^1  -   _il?_Jl_i^IL,       *  5.5  X  10"''^  grams.     This"  value  is  negligible  in 
"     1.6  x"l0-3^ 


88 

comparison  with  unity  or  /^THH  .     The  equation  For  the  mean  free 

path  of  the  electrons  is, 

/  -  5 — cms. 

In  this  equation        is  th-^  number  of  molecules  per  cubic  centimeter. 
From  the  kinetic  theory  of  gases        may  be  calculated, 
r  V  =  R  T  the  gas  law  which  becomes  p  V  =  2/3  ^ =  2/3  N  1/2  mO^. 
Where^is  the  average  total  kinetic  energy. 

i  8  the  number  of  molecules  in  ■''"olume  V, 
is  the  square  of  average  velocities  of  molecules, 
1/2  mC^  -      T, where  o(  is  the  universal  constant, 
?  V  =  2/3  i:oCT 

or, 

?  =  2/3  :ioC7  for  unit  volume, 

IT  =  3/2  -JL,    the  number  of  molecules  per  unit  volume, 

,      13,6  X  980  h  .  .    .  . 

=  3/2  — r  —  )  h  IS  m  cm, 

2  X  10**^°  T 


Suppose  T  -  27  +  273  ^  300  for  this  Tork, 
15.6  X  980 
2  X  10""!^  X  300 


N  =  3/2  i^'^.^,-,  h  =  33  X  10^^  h  in  cm.  mercury. 


Substitute  this  value  for  expression  for  mean  free  path, 

^    ^  1  _    955  X  10-5 


TT  X  5:5  X  lois  h  lo-is 

Applying  the  equation  W  =  E  t , 
e    =  4.67  X  10-10  ij.^   c^^  Tj. 


cm. 


300  D 

965  X  10-5 
 ••—   cm. 


For  h  =  0.5  cm.     D  =  1.0  cm.     V  -  330  volts, 

w      ii-67  X  10-^Q  9  65  X  10-^  , 

^  =       "^0~0'~x  1,0    x"^,T         =  1-^^  ^  .10 


29 

For  an  average  of  ten  rou-hly  det'='rrr,ined  valuo3  1.05  x  lO"^^  ergs 
was  o>?tai!:?>d  for  the  ri.iwirriUia  energy  required  to  produce  an  ion  in 
hydrogen  gaa  ly  collision. 

b)  Second  assumption  regarding  mean. free  path  of  elec':rons. 

If  it  be  assumed  that  the  negative  iona  or  electrons  occupy  no 
space  in  the  gas,  or,  thit  the  hydrogen  gas  -^cts  as  though  the  elec- 
trons '.vere  not  present,   then  the  value  of  the  mean  free  path  is  the 
same  as  th-.t  of  the  molecules.     That  is, 


TT  N  (T^'f^  h 

-  685  X  10-5 

 cm. 

h 

For  h  =  0.5  cm."    D  =  1.0  cm.     V  =  330  volts, 

4.67  X  10"^5  X  685  x  10*"^  x  330 
^  "  "       300  x'oTs  =  •'^0  X  lO'll  ergs. 

About  40^  increase  from  1,09  x  10""^^  ergs, 
c)  Third  assumption  regarding  mean  free  path  of  electrons. 

Ey  using  the  assumption  that  the  mean  free  path  of  the  electrons 
is  47^_  times  mean  free  path  of  ^he  hydrogen  molecules,  as  Bishop  di4 
the  following  results  are  obtained. 

TT  33  x  lO^T^h  X  4  xTo^  ~  ^ 

^  ^  4^Z^0z^xA36Q..jLJLai5  „  .        ^  ..-15  V 

300  x  j3      -  21  x  10  ^ 

For  P  =  0,3  cm.     D  =  1,0  cm.    V  =  260  volts. 

T  =  21  X  10-15  ^  ^^32  ^  ^Q^ll  ^ 

1x0.3 

An  average  of  10  calculations  gives  1.^7  x  10~'l  ergs. 

To  recapitulate,   the  values  det^rmi^ed  above  and  those  by  other 

inves tig?. tors  are:- 

1.-  For  an  isolate  molecule  the  theoretical  value  is  2,18  x  10"^^ 
ergs. 


00 

2.  -  For  a  molecule  in  an  slectric  field  the  theoretical  value  is 

1.63  X  10""^^  erga. 

3.  -  The  average  of  ten  vali^eo  determined  from  the  data  in  a-^cord- 

ance  with  the  first  assumption  is  1,05  x  10""^^  erga. 

4.  -  The  vTlue  determined  in  accordance  vith  the  second  assumption 

is  0.70  X  10"^^  ergs, 

5.  ~  The  average  of  ten  values  deterrair.ed  in  accordance  v;ith  the 

third  assumption  is  1.77  x  lO""-^-*-  ergs» 

6.  -  Bishops  obtained  a  value,        a  method  sirailar  to  that  used  in 

this  work  and  in  accordance  'A'ith  the  third  assumption  used 
in  the  '"ifth  determination,  1.58  x  10""-^  ergs, 

7.  -  Rutherford*^  determined  the  energy  required  to  produce  an  ion  by 

the  alpha  particle.    His  value  is  2,7  x  10"" -^-^  ergs, 

8.  -  Ceiger"^,  and  later  Taylor  ,  using  the  same  method  obtained  about 

5  X  10~^^  ©rgs,  ani  other  investigators  obtained  values 

even  as  large  as  10  x  10"^^  ergs. 
In  the  me*-hod,  based  on  the  ionizing  poTver  of  alpha  particles, 
it  is  assumed  th-^t  their  kinetic  energy  is  entirely'-  transformed  into 
energy  of  ionization,  and  that  the  decrease  of  the  kinetic  energy 
over  a  certain  range  divided  by    the     total  numbers  of  ions  produced 
gives  the  energy  required  to  produce  one  ion,     flince  a  part  of  the 
kinetic  energy  of  the  alpha  particle  increases  the  average  kinetic 
energy  of  the  gas  without  producing  ions,   the  ionizing  energy  is 
taken  too  large  and  th-?  energy  required  to  produce  an  ion  is  neces- 
sarily too  large, 

1.  Phil.  Rev.  p,325,  TTov,  1911. 

2,  Radio-Ac'-ivity,   second  edition,  p.  552, 

3,  Proc.  Royal  "^.oc .  "^xl.  82,  p. 486,  1S09, 

4.  Phil,::ag,  P.G7C,  April,  1912, 


Value  r!UirV»er  (l)  r  <*pros'='r  ts  the  rninimuru  energy  required  to  pro- 
duce an  ion  v...cii  a  molecuiLO  is  isolated.     Thio  value  is  niuch  larger 
than  that  for  a  molecule  in  an  electric  field.    Ru therf ord!s^  deterrrdn- 
ation  is  much  nearer   the  vnlue  nurr.ber  (l)   than  any  of  the  others. 

Pi  shop's  determination  is  very  close  to  the  value  number  (2), 
while  my  determination  of  the  ionizing  energy  is  slightly  larger. 
Since  the  assumption  regarding  the  mean  free  path  in  number  5  and  6 
are  the  3^-r..e,  and  these  values  differ  very  alic^htly  from  the  theo- 
retical value  number  (2),  it  indicates  that  the  assumptions  made  are 
not  far  from  Lhe  truth. 

Owing  to  lack  of  time  a.....  c^i^ce  an  exact  determination  of  the 
minimum  energy  required  to  produce  an  ion  by  collision  is  impossible; 
in  the  near  future  hov;ever,   it  is  hoped  that  this  can  be  done  with 
the  data  already  in  hand. 

Design  of  a  sensitive  photo-electric  cell. 
At  the  beginning  of  this  investigation  it  was  expected  to  de- 
sign a  photo-electric  cell  which  could  be  used  in  astro-photometric 
investigations.     A  tube  has  been  constructed  which  has  combined  in 
its  design  as  many  of  the  most  favorable  conditions  for  sensitiveness 
as  is  practical,  but  owing  to  lack  of    tim.e  it  has  not  been  tried  out. 

The  best  angle  of  incidence  is  about  60°,   the  best  hydrogen  gas  pres- 

and 

sure  is  about  2  mm.,^the  best  metal  for  practical  use  is  potassium  at 
about     25°cr  .    The  diagram,   Fig.  9   below  shows  the  plan  of  the  tube. 
The  diameter  of  the  tube,   Fig,    9    is  about  2  cm.  and  the  end  E  is 
sealed  as  uniformly  anJ  as  nearly  plane  as  po^^sible.     The  incident 
light  enters  through  E  and  is  incident  upon  the  cylindrical  metal 
cathode  C,         ;:he  end  of  which  is  turned  to  A  G0°  cone.     Upon  the 

1.  Radio-Activi ty,   second  edition,  p,  552, 


I 


shaded  portion  of  C  is  uiatilled  a  layer  of  potasaium  metal  -md  the 
hydride  surface  formed.     This  is  done  bef'ore  th-  cathode  is  moved 
into  its  present  position  Ty    means  of  an  electrorcagnet. 

The  anode  A  is  a  hollow  cylinder  with  the  upy-er  sides  making 
C0°  with  each  oth^-^r  so  thnt  the  shortest  distance  between  all  pointfl 
on  the  surface  of  C  are  equidistant  from  the  inner  surface  of  A,  the 
inner  surface  being  highly  polished  or  8ilvf>red  in  order  to  reflect 
back  to  the  cathode  the  rays  ;vhich  are  first  reflected  from  it.  In 
this  way  multiple  reflection  is  obtained  with  a  large  anode  surface 
for  collecting  the  ions  pro^-uced  by  the  action  of  the  electrons 
emitted  from  the  cathode  Then  light  is  incident  upon  it.     It  is 
hopexi  that  this  tube  with  the  most  sensitive  conditions  now  known, 
will  be  sensitive  encugh  to  take  the  place  of  the  erratic  selenium 
cell  used  in  astronomical  work. 


E 


A 


FIO.  9 


0  3 

Summary  and  ConclueioriS, 

The  follovring  facts  ar?  established  by  this  investigation  for 
this  type  of  photo-electric  cell. 

1.  -    Owing  to  the  low  melting  temperature  of  caesium  the  use  of  this 

metal  in  photo-electric  cells  for  photometric  use  is  very  im- 
prac  tical . 

2.  -    The  temperature  at  ^vhioh  it  is  best  to  operate  a  potassium  c-^^ll 

is  about  c5°C. 

3.  -    Cooling  the  potassium  cell  much  below  25°G  does  not  increase 

its  8(=»nsit iveness. 

4.  ~      The  sensibility  of  a  potassium  cell  can  be  increased  more  than 

100  times  by  the  process  of  forming  the  hydride  surface. 

5.  --    The  distance  bef.veen  the  electrodes  for  best  sensitiveness  is 

about  0. 5  cm, 

6.  -    The  hydrogen  gas  pressure  at  V7hich  the  cell  is  most  sensitive 

lies  bet'.'-een  2  and  3  mm.  of  mercury, 

7.  -    The  potential  difference  applied  to  the  electrodes  for  most 

sensitive  conditions  is  about  330  volts, 

8.  -    The  minimum  energy  required  to  produce  an  ion  by  collision  was 

calculated  from  the  data  and  found  to  be  of  the  order  1,77  x 
1C~^1  ergs,  while  the  theoretical  value  determined  is  1.53  x 
10-11  ergs. 

9.  -    Assuming  that  the  straight  lines  obtained  which  show  the  rela- 

tion between  current  and  intensity  of  illumin-.tion  hold  for  ex- 
ceedingly small  intensities,   th'^n  by  u«ing  a  tilted  electro- 
scope of  sensibility  lO^lS  amperes,  a  candle  could  be  detected 
at  a  distance  of  2.7  mdles.     This  indicates  that  it  is  highly 
I- robable  th-'.t  a  photo-electric  cell  could  be  used  in  astro- 
photometric  work. 


04 

The  author  takes  great  pleasure  in  acknowledging  his  indebted- 
nesa  to  rrofeoscr  A.  ?.  Carman  for  the  facilities  for  this  investi- 
gation, and  to  TrofeGoor  Jakob  Kunz,  both  for  his  general  supervision 
of  the  worTc  and  for  many  valuable  sufygeations 


I 


Rcholaatic  Record  of  Jacob  Harrett  Temp, 

The  author  was  born  Au.t^ust  C6,   1877,   in  Haltiroore,  Ud.,   .vhere  he 
obtained  his  early  education  in  the  public  schools.     On  October  15, 
1900  he  entered  "The  Deichmann  college  Preparatory  r)Chool"of  Balti- 
more, Md,,  where  he  was  graduated  in  1903, 

During  the  years  1902-1006  he  attended  the  University  of  Illi- 
nois, submitted  a  thesis  on  "Apparatus  and  Methods  for  Measuring 
Electric  Wa-.^es",  and  received  the  degree  A.^^.  in  flcience.  During 
the  years  1906-1908  he  was  assistant  in  Physicsf  at  Purd\ie  University, 
Lafayette,  Indiana.     During  the  years  1908-1911  he  was  assistant  in 
Physics  at  the  University  of  Illinois,  and  at  the  same  time  completed 
graduate  courses  in  Experimental  and  Theoretical  Physics,   and  Mathe- 
n-.atics.     In  1910  he  submitted  a  thesis  on  "The  Magnetic  Properties 
of  Certain  Rare  Eartiib",  as  partial  fulfillment  for   the  requirements 
for  the  degree  of  A.M.  in  Physics, 

For  the  year  1911-1912  he  was  awarded  a  fellowship  in  Physics, 
and  during  this  time  he  completed  the  thesis  on  "The  Conditions  of 
Sensibility  of  Pho to-Elec tr ic  Cells  with  Alkali  Metals  and  Hydrogen". 

A  paper  in  press,   title,-  "A  Substitute  for  a  Eronson  Resist- 
ance," by  Cornelius  Kemp, 


